1. Field of the Invention
The present invention relates to a silicon single crystal wafer having few crystal defects in which nitrogen is doped, as well as to a method for producing such a silicon single crystal wafer.
2. Description of the Related Art
Along with a decrease in size of semiconductor devices for achieving an increased degree of integration of semiconductor circuits such as DRAM, more severe quality requirements have recently been imposed on silicon single crystals which are grown by the Czochralski method (hereinafter referred to as the CZ method) for use as materials for substrates of semiconductor devices. Particularly, there has been required a reduction in density and size of grown-in defects such as flow pattern defects (FPDs), laser scattering tomography defects (LSTDs), and crystal originated particles (COPs), which are generated during the growth of a single crystal and degrade oxide dielectric breakdown voltage and characteristics of devices.
In connections with the above-mentioned defects incorporated into a silicon single crystal, first are described factors which determine the concentration of a point defect called a vacancy (hereinafter may be referred to as V) and the concentration of a point defect called an interstitial (hereinafter may be referred to as I).
In a silicon single crystal, a V region refers to a region which contains a relatively large number of vacancies, i.e., depressions, pits, voids or the like caused by missing silicon atoms; and an I region refers to a region which contains a relatively large number of dislocations caused by excess silicon atoms or a relatively large number of clusters of excess silicon atoms. Further, between the V region and the I region there exists a neutral (hereinafter may be referred to as N) region which contains no or few excess or missing silicon atoms. Recent studies have revealed that the above-mentioned grown-in defects such as FPDs, LSTDs, and COPs are generated only when vacancies and/or interstitials are present in a supersaturated state and that even when some atoms deviate from their ideal positions, they do not appear as a defect so long as vacancies and/or interstitials do not exceed the saturation level.
It has been confirmed that the concentration of vacancies and/or interstitials depends on the relation between the pulling rate (growth rate) of crystal in the CZ method and the temperature gradient G in the vicinity of a solid-liquid interface of a growing crystal, and that another type of defect called oxidation-induced stacking fault (OSF) is present in ring-shape distribution in the N-region between the V region and the I region.
The manner of generation of defects due to growth of a crystal changes depending on the growth rate. That is, when the growth rate is relatively high; e.g., about 0.6 mm/min or higher, grown-in defects such as FPDs, LSTDs, and COPs-which are believed to be generated due to voids at which vacancy-type points defects aggregate are present at a high density over the entire radial cross section of a crystal. The region where these defects are present is called a "V-rich region". When the growth rate is not greater than 0.6 mm/min, as the growth rate decreases the above-described OSF ring is generated from a circumferential portion of the crystal. In such a case, L/D (large dislocation, simplified expression of interstitial dislocation loop) defects such as LSEPDs and LFPDs--which are believed to be generated due to dislocation loop--are present at a low density outside the OSF ring. The region where these defects are present is called an "I-rich region". Further, when the growth rate is decreased to about 0.4 mm/min, the above-described OSF ring converges (shrinks) to the center of a wafer and disappears, so that the I-rich region spreads over the entire cross section of the wafer.
Further, there has been recently found the existence of a region, called a N (neutral) region, which is located between the V-rich region and the I-rich region and outside the OSF ring and in which there exists neither defects of FPDs, LSTDs and COPs stemming from voids; defects of LSEPDs and LFPDs stemming from a dislocation loop; nor OSF. The region has been reported to be located outside the OSF ring, and substantially no oxygen precipitation occurs there when a single crystal is subjected to a heat treatment for oxygen precipitation and the contrast due to oxide precipitates is observed through use of an X-ray beam. Further, the N-region is on an I-rich region side, and is not rich enough to cause formation of LSEPDs and LFPDs.
It is confirmed that N-region is present also inside of the OSF ring, where neither defects due to vacancy, defects due to dislocation loop, nor OSF is present.
Since these N-regions are generally diagonal to a growing axis when the growing rate is decreased, they are present only in a part of the surface of the wafer.
As to the N-regions, Voronkov theory (V. V. Voronkov: Journal of Crystal Growth, vol. 59, p.625-643, 1982) proposes that a total density of point defects depends on a value of F/G, which is a ratio of a pulling rate (F) to an intra-crystal temperature gradient along the pulling direction (G). According to the theory, the pulling rate must be constant in the surface and G is distributed in the surface. Therefore, at a certain pulling rate, there is obtained only a crystal having V-rich region in its center portion, I-rich region in the periphery of V-rich region, and N-rich region between the above two regions.
To solve the above-mentioned problem that N-region is present only diagonally, recently an improvement of distribution of G in the surface has been made. As a result, it has become possible to produce a crystal wherein N-region is expanded horizontally in the entire surface at a certain pulling rate, when the crystal is pulled with gradually lowering a pulling rate F. The region of the crystal where N-region is expanded in the entire surface can be enlarged axially to some extent, by pulling the crystal at a pulling rate at which N-region is expanded horizontally. Further, considering that G varies as the crystal grows, it has been proposed that the N-region can be expanded to some degree in the entire wafer surface along direction of growth when a ratio F/G is controlled to be constant by controlling a puling rate.
Meanwhile, it has been known that defects in FZ silicon is decreased in a silicon single crystal in which nitrogen is doped. Such method is also applied to CZ method, using the unique oxygen precipitation characteristics or the like.
However, for producing such a single crystal that the N-region having a very low defect density is expanded to the entire crystal, the pulling rate must be controlled minutely within an extremely narrow range, and the efficiency of furnace of apparatus for growing crystal (hot zone: HZ) is limited. Therefore, it has been difficult to expand the N-region to an axial direction of crystal.
Accordingly, the yield of the crystal wherein N-region is expanded to the entire crystal was low, and it was difficult to maintain the quality of crystal.
It has been believed that quality of a general CZ crystal (having V-rich region in its most surface) in which nitrogen is doped is good, since almost no grown-in defect is observed apparently. However, a detailed analysis revealed that there were a lot of small defects although aggregation of defect was suppressed by nitrogen doping. Besides, oxide dielectric breakdown voltage is not so good. Furthermore, when nitrogen is doped in high concentration to eliminate defects, there arise defects such as OSF due to oxygen precipitation caused by nitrogen in heat treatment in a device process or the like.